Many circuits are nowadays operated using radio-frequency signals, in particular in the field of digital technology. The circuits may in this case be in the form of integrated circuits, circuits of modular design or circuits in a discrete form, for example on printed circuit boards. In addition to the desired function of a circuit, the radio-frequency signals often also result in undesirable effects. This includes the outputting of radio-frequency signals which are created in the circuit and are inadvertently emitted to the surrounding area. This effect is referred to as electromagnetic interference (EMI). In particular, the expression electromagnetic interference is used when the emission of electromagnetic fields results in a negative influence on the operation of this particular circuit, or of other circuits. This can even lead to destruction of a circuit.
The measure of how specifically a circuit can be influenced electromagnetically is defined by the electromagnetic compatibility (EMC). The EMI level should in this case be below specific EMC limit values for the circuit, as a function of the frequency. As a function of the frequency in this case means that compliance with the limit values is checked by analysis of the frequency spectrum of the emitted signal. The EMI of a circuit should thus be reduced to such an extent that it is below the respective limit value at every frequency. When reducing the EMI, it is therefore also possible to speak of reducing the magnitude of an emitted signal in one portion of the frequency spectrum.
Possible methods for this purpose are shielding, in which the emission of the electromagnetic fields is attenuated, or deliberately designing a circuit in such a manner that the components are arranged in such a way that the emitted electromagnetic fields are as weak as possible. However, it is not always possible to take this into account in the design of a circuit.
High-speed digital circuits are particularly susceptible to EMI problems. In order to reduce the EMI in these circuits, it is possible to use a method which is referred to as “spread spectrum clocking”. In this method, the frequency of the system clock for a circuit is slowly modulated, so that the frequencies of the emitted electromagnetic fields also vary. Slow variation of the frequency of the system clock likewise results in a slow variation of the frequency of the peaks in the frequency spectrum of the emitted electromagnetic field which is created during operation of the circuit. Averaged over time, the peaks in the frequency spectrum are thus distributed over a wider frequency range, and the amplitude of the peaks is decreased overall. This can be referred to as smearing of the spectral energy. The manner of the distribution over a wider frequency range depends on the way in which the frequency of the system clock is varied. In some circumstances, however, it is possible for a situation to occur in which influencing the system clock also results in an offset in the time control, which can have a disadvantageous effect on other parts of a digital system. In particular, it is possible to disadvantageously influence the operation of phase locked loops, PLLs.
It may therefore be desirable to provide a circuit arrangement and a method for reducing electromagnetic interference, while at the same time maintaining a system clock at a stable frequency.